Fluid operated actuators convert a fluid pressure to a work piece using an actuator that typically consists of a piston in a cylinder. Although there are various suitable fluids that may be used, the fluid applied to the actuator generally comprises pneumatic or hydraulic fluid, for example. Pneumatic operated actuators are generally used where the compressibility of air is desired or to obtain much higher flow rates and thus faster response times while hydraulic operated actuators are generally employed when high actuating forces are required. Both fluids have advantages and in some situations, either pneumatic or hydraulic fluid may be used.
The position of the fluid operated actuator is usually controlled with a group of devices referred to in the art as ‘positioners.’ The positioners are designed to control the position of the fluid operated actuator in response to a user/operator input or a change in the environmental conditions, such as a change in the load acting on the actuator. Existing products commonly found on the market include positioners of a wide spectrum of levels of sophistication and complexity, ranging from positioners that use simple mechanical feedback mechanisms to control the position of the actuator, to more complex positioners that incorporate a combination of mechanical, electronic, and software technologies to provide a higher capability of control.
The particular positioner employed generally depends on two performance criteria, namely, speed and stability. A significant problem with existing designs is that a compromise must be made between these two performance criteria. As the speed of the actuation is increased, the stability of the control system is decreased until the control system becomes totally unstable. The speed of actuation is largely dictated by the quantity of fluid flow that can be provided by the positioner. One established solution to this problem is to add external boosters to a positioner and actuator positioning system. These boosters provide much higher fluid flow levels to the actuator, thus speeding up the response of the system. However, the increase in speed often increases cost and complexity of the additional boosters along with requiring additional pipe work.
Another problem with existing products relates to the sensitivity of the control system to changes and non-linearities within components contained within the control valve that is internal to the positioner. These changes may be caused by environmental changes such as temperature and orientation, or changes over time such as drift and wear. The non-linearity of components may be caused by friction amongst other factors. To combat these changes and non-linearity problems, additional complexity and cost is usually necessary within the algorithms of the positioner.
The present invention enables positioners to be made that have speeds of actuation that approach or exceed systems fitted with external boosters, while maintaining stability without the additional cost and complexity of the boosters.
The present invention overcomes the problems associated with changes and non-linearity within components contained within the control valve that is internal to the positioner.
Aspects
According to an aspect of the invention, a positioner for controlling a fluid operated actuator including a first fluid chamber and a second fluid chamber (110) comprises:                a first fluid conduit coupled to the first fluid chamber;        a second fluid conduit coupled to the second fluid chamber; and        a differential pressure controller adapted to control a fluid supply to the first and second fluid conduits based on a differential pressure between the first and second fluid chamber.        
Preferably, the positioner further comprises a differential pressure sensor adapted to measure a differential pressure between the first and second fluid chambers.
Preferably, the positioner further comprises a fluid control valve including:                a fluid inlet coupled to a pressurized fluid source;        a first fluid outlet coupled to the first fluid conduit; and        a second fluid outlet coupled to the second fluid conduit.        
Preferably, the positioner further comprises a position sensor coupled to the actuator.
Preferably, the differential pressure controller is further adapted to generate a desired differential pressure based on a desired actuator position and a measured actuator position.
Preferably, the differential pressure controller is further adapted to compare the desired differential pressure to a measured differential pressure.
Preferably, the positioner further comprises a diaphragm assembly coupled to the actuator including:                a pilot chamber;        a first biasing chamber; and        a second biasing chamber.        
Preferably, the pilot chamber comprises a pilot inlet and a pilot diaphragm.
Preferably, the first biasing chamber comprises a first biasing diaphragm and a first fluid port coupled to the first fluid conduit and wherein pressure within the first fluid conduit acts on the first biasing diaphragm to bias the diaphragm assembly in a first direction.
Preferably, a fluid control valve is actuated to a first position when the fluid pressure within the first biasing chamber reaches a threshold value.
Preferably, the second biasing chamber comprises a second biasing diaphragm and a second fluid port coupled to the second fluid conduit and wherein pressure within the second fluid conduit acts on the second biasing diaphragm to bias the diaphragm assembly in a second direction.
Preferably, a fluid control valve is actuated to a second position when the fluid pressure within the second biasing chamber reaches a threshold value.
Preferably, the diaphragm assembly is coupled to a fluid control valve.
According to another aspect of the invention, a positioner for controlling a fluid operated actuator, including a first fluid chamber and a second fluid chamber, the positioner comprises:                a fluid control valve;        a first fluid conduit coupled to a first outlet of the fluid control valve and the first fluid chamber;        a second fluid conduit coupled to a second outlet of the fluid control valve and the second fluid chamber; and        a diaphragm assembly coupled to the fluid operated actuator, the first fluid conduit, the second fluid conduit, and the fluid control valve, wherein the diaphragm assembly is adapted to control the fluid control valve based on a differential pressure between the first fluid conduit and the second fluid conduit.        
Preferably, the diaphragm assembly comprises:                a pilot chamber;        a pilot diaphragm; and        a pilot inlet adapted to receive a pilot pressure to pressurize the pilot chamber and bias the pilot diaphragm.        
Preferably, the diaphragm assembly comprises:                a first biasing diaphragm; and        a first biasing chamber including a first fluid port coupled to the first fluid conduit and wherein pressure within the first fluid conduit acts on the first biasing diaphragm to bias the diaphragm assembly in a first direction.        
Preferably, the control valve is actuated to a first position when the pressure in the first biasing chamber reaches a threshold value.
Preferably, the diaphragm assembly comprises:                a second biasing diaphragm; and        a second biasing chamber including a second fluid port coupled to the second fluid conduit and wherein pressure within the second fluid conduit acts on the second biasing diaphragm to bias the diaphragm assembly in a second direction.        
Preferably, the control valve is actuated to a second position when the pressure in the second biasing chamber reaches a threshold value.
Preferably, the positioner further comprises a position sensor coupled to the actuator.
Preferably, the positioner further comprises a differential pressure sensor adapted to measure a differential pressure between the first and second fluid chambers.
According to another aspect of the invention, a method for controlling a fluid operated actuator including a first fluid chamber and a second fluid chamber comprises the steps of:                generating a desired differential pressure between the first and second fluid chambers; and        controlling a fluid supply to at least one of the first or second fluid chambers to control a position of the fluid operated actuator based on the desired differential pressure.        
Preferably, the method further comprises the steps of:                measuring a differential pressure between the first and second fluid chambers; and        actuating a fluid control valve if the difference between the measured differential pressure and the desired differential pressure exceeds a threshold value.        
Preferably, the step of supplying fluid pressure to at least one of the first or second fluid chambers comprises adjusting a pilot pressure supplied to a diaphragm assembly coupled to the actuator to control the differential pressure between the first and second fluid chambers.
Preferably, the differential pressure between the first and second fluid chambers controls a position of a fluid control valve.
Preferably, the method further comprises the steps of:                measuring a position of the actuator;        comparing measured actuator position to a desired actuator position, and        generating a new desired differential pressure if the difference between the measured actuator position and the desired actuator position exceeds a threshold value.        